Knowledge Building: Turbochargers

Turbochargers are often misconstrued as devices for producing greater engine power. But the turbocharger is, in fact, a device that is used to enhance the efficiency of the engine by supplying a pressurized charge of air to the engine’s cylinders.

This dramatically increases the oxygen density of the air entering the cylinder. With the higher oxygen content, a more complete fuel burn is achieved. This, in turn, makes the engine produce more power, but more importantly, the vehicle is more “emissions-friendly.”

The turbocharger is an exhaust-driven air pump that is usually mounted directly on the exhaust manifold. It is important to note that the turbocharger is driven mainly by kinetic energy (heat) as well as exhaust gas pressure.

The turbocharger assembly can be broken down into four main sections: the turbine side, the compressor side, the waste gate mechanism, and the Centre Housing/Hub Rotating Assembly (CHRA). The CHRA houses the bearings, the bearing housing and the common shaft to which both the turbine and the impeller are directly connected.

The turbine housing contains the turbine wheel and is subjected to exhaust gas at very high temperatures. For this reason, the turbine housing has to be able to withstand heat and oxidation. It is made from austenite, a crystalline structure made up of iron and carbon.

The compressor impeller is mounted at the opposite end of the turbine wheel and is used to pull fresh air in from the air cleaner and divert it through the compressor housing to charge the intake manifold and the engine’s cylinders. Just like the turbine

wheel, due to the high-speed rotation, the blade sections and profile have to be very precisely balanced to prevent failure.

A waste gate is used to control overboosting, which would cause cylinder pressures to become too high. The waste gate is used to make the engine more efficient in all ranges. In essence, the waste gate allows the turbocharger to take on two roles, by allowing it to perform like a high-flow turbo at low torque and low speeds, and perform like a low-flow turbo at high torque and high speeds. This feature helps to reduce turbo lag. The waste gate diverts the exhaust gas away from the turbine wheel to the exhaust pipe.

Most modern turbochargers utilize specially designed plain bearings both in the radial as well as the axial thrust bearing assembly. The radial bearings are either stationary plain bearing bushings or rotating double plain bushings. Due to the extreme speeds involved, roller bearings are not used for small turbochargers. The rotating bearings are designed to rotate slower than the shaft itself — usually at about one-third of the shaft’s speed. Both styles use a film of oil for the shaft to ride on, which means that it is “hydro-dynamically suspended.” A thrust bearing is used to set and maintain the axial play of the turbine shaft. Thrust washers are located at the compressor end of the shaft assembly.

The turbo shaft runs through the centre of the turbocharger, and is used to mount the turbine wheel on the exhaust side of the turbo and the impeller on the compressor side. The turbine wheel can be bolted on but is usually friction-welded to the shaft. The impeller is bolted on. The shaft is made from a steel alloy and its bearing journals are induction-hardened and ground for dimensional accuracy.

Even though the turbocharger is responsible for producing the boost that is required by the engine, without other parts that work in conjunction with the turbocharger, it would be inefficient and do more harm than good.

After the turbocharger compresses the air coming in from the air filter, the air becomes heated due to the compression. This means it has less oxygen per volume of air, due to the expansion of the air reducing the density. The charge air cooler (CAC) is used to lower the temperature of the compressed air from the turbocharger. This system uses outside air rushing through the cooler to lower the charged air temperature. Other types of charged air coolers include the intercooler, which uses the engine coolant to cool the compressed air. This is similar to the oil cooler on the engine. These coolers have a very low cooling capability because they are limited by the cooling medium, which is the engine coolant. However, in an application where there is limited air flow, this is better than nothing at all.

One common problem that occurs with a turbocharged engine is a low power complaint caused by low boost. This can be caused by something as simple as a loose clamp. Checking for a boost leak can be done with a smoke machine or pressurizing the intake system then leak testing with a water-soap solution.

It is important to be sure customers understand that there is an idle down period after an extended use. This allows the turbocharger to return to its proper idling temperature before oil flow is removed, thus preventing shaft damage. This is explained in the owner’s manual as well.